optimize sketch reset

[bitfaster]

Unroll the reset loop to improve throughput. Not earth shattering, but worth almost 1 ms for a 1 million element table.

Method	Size	Mean	Error	StdDev	Ratio
Reset1	4	8.893 ns	0.0390 ns	0.0304 ns	1.00
Reset2	4	6.170 ns	0.0873 ns	0.0816 ns	0.70
Reset4	4	5.883 ns	0.0722 ns	0.0675 ns	0.66
Reset1	128	282.571 ns	2.4478 ns	2.2896 ns	1.00
Reset2	128	176.850 ns	0.7743 ns	0.6045 ns	0.63
Reset4	128	160.341 ns	0.9347 ns	0.7806 ns	0.57
Reset1	8192	17,454.963 ns	84.2848 ns	74.7163 ns	1.00
Reset2	8192	10,810.292 ns	36.0139 ns	30.0733 ns	0.62
Reset4	8192	10,252.838 ns	36.5366 ns	30.5097 ns	0.59
Reset1	1048576	2,258,716.657 ns	7,187.0479 ns	6,371.1308 ns	1.00
Reset2	1048576	1,481,100.085 ns	28,584.5228 ns	28,073.8235 ns	0.66
Reset4	1048576	1,391,683.608 ns	13,086.8858 ns	11,601.1834 ns	0.62